Not applicable.
The present invention is directed generally to the transmission of information in communication systems and components. More particularly, the invention relates to communication systems and components which compensate for distortion of signals being transmitted by the systems and components.
The development of digital technology provided resources to store and process vast amounts of information. While this development greatly increased information processing capabilities, it was soon recognized that in order to make effective use of information resources, it was necessary to interconnect and allow communication between information resources. Efficient access to information resources requires the continued development of information transmission systems to facilitate the sharing of information between resources.
The continued advances in information storage and processing technology has fueled a corresponding advance in information transmission technology. Information transmission technology is directed toward providing high speed, high capacity connections between information resources. One effort to achieve higher transmission capacities has focused on the development of optical transmission systems for use in conjunction with high speed electronic transmission systems. Optical transmission systems employ optical fiber networks to provide high capacity, low error rate transmission of information over long distances at a relatively low cost.
The transmission of information over fiber optic networks is performed by imparting the information in some manner to a lightwave carrier by varying the characteristics of the lightwave. The lightwave is launched into the optical fiber in the network to a receiver at a destination for the information. At the receiver, a photodetector is used to detect the lightwave variations and convert the information carried by the variations into electrical form.
In most optical transmission systems, the information is imparted by using the information data stream to either modulate a lightwave source to produce a modulated lightwave or to modulate the lightwave after it is emitted from the light source. The former modulation technique is known as xe2x80x9cdirect modulationxe2x80x9d, whereas the latter is known as xe2x80x9cexternal modulationxe2x80x9d, i.e., external to the lightwave source. External modulation is more often used for higher speed transmission systems, because the high speed direct modulation of a source often causes undesirable variations in the wavelength of the source. The wavelength variations, known as chirp, can result in transmission and detection errors in an optical system.
Data streams can be modulated onto the lightwave using a number of different schemes. The two most common schemes are return to zero (RZ) and non-return to zero (NRZ). In RZ modulation, the modulation of each bit of information begins and ends at the same modulation level, i.e., zero, as shown in FIG. 1a. In NRZ schemes, the modulation level is not necessarily returned to a base modulation level, i.e., zero, at the end of a bit, but is directly adjusted to a level necessary to modulate the next information bit as shown in FIG. 1b. Other modulation schemes, such as duobinary and PSK, encode the data in a waveform, such as in FIG. 1c, prior to modulation onto a carrier.
In many systems, the information data stream is modulated onto the lightwave at a carrier wavelength, xcexo, (FIG. 2a) to produce an optical signal carrying data at the carrier wavelength, similar to that shown in FIG. 2b. The modulation of the carrier wavelength also produces symmetric lobes, or sidebands, that broaden the overall bandwidth of the optical signal. The bandwidth of an optical signal determines how closely spaced successive optical signals can be spaced within a range of wavelengths.
Alternatively, the information can be modulated onto a wavelength proximate to the carrier wavelength using subcarrier modulation (xe2x80x9cSCMxe2x80x9d). SCM techniques, such as those described in U.S. Pat. Nos. 4,989,200, 5,432,632, and 5,596,436, generally produce a modulated optical signal in the form of two mirror image sidebands at wavelengths symmetrically disposed around the carrier wavelength (FIG. 2c). Generally, only one of the mirror images is required to carry the signal and the other image is a source of signal noise that also consumes wavelength bandwidth that would normally be available to carry information. Similarly, the carrier wavelength, which does not carry the information, can be a source of noise that interferes with the subcarrier signal. Modified SCM techniques have been developed to eliminate one of the mirror images and the carrier wavelength, such as described in U.S. Pat. Nos. 5,101,450 and 5,301,058.
Initially, single wavelength lightwave carriers were spatially separated by placing each carrier on a different fiber to provide space division multiplexing (xe2x80x9cSDMxe2x80x9d) of the information in optical systems. As the demand for capacity grew, increasing numbers of information data streams were spaced in time, or time division multiplexed (xe2x80x9cTDMxe2x80x9d), on the single wavelength carrier in the SDM system as a means to provide additional capacity. The continued growth in transmission capacity has spawned the transmission of multiple wavelength carriers on a single fiber using wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d). In WDM systems, further increases in transmission capacity can be achieved not only by increasing the transmission rate of the information via each wavelength, but also by increasing the number of wavelengths, or channel count, in the system.
There are two general options for increasing the channel count in WDM systems. The first option is to widen the transmission bandwidth to add more channels at current channel spacings. The second option is to decrease the spacing between the channels to provide a greater number of channels within a given transmission bandwidth. The first option currently provides only limited benefit, because most optical systems use erbium doped fiber amplifiers (xe2x80x9cEDFAsxe2x80x9d) to amplify the optical signal during transmission. EDFAs have a limited bandwidth of operation and suffer from non-linear amplifier characteristics within the bandwidth. Difficulties with the second option include controlling optical sources that are closely spaced to prevent interference from wavelength drift and nonlinear interactions between the signals.
A further difficulty in WDM systems is that chromatic dispersion, which results from differences in the speed at which different wavelengths travel in optical fiber, can also degrade the optical signal. Chromatic dispersion is generally controlled in a system using one or more of three techniques. One technique to offset the dispersion of the different wavelengths in the transmission fiber through the use of optical components such as Bragg gratings or arrayed waveguides that vary the relative optical paths of the wavelengths. Another technique is intersperse different types of fibers that have opposite dispersion characteristics to that of the transmission fiber. A third technique is to attempt to offset the dispersion by prechirping the frequency or modulating the phase of the laser or lightwave in addition to modulating the data onto the lightwave. For example, see U.S. Pat. Nos. 5,555,118, 5,778,128, 5,781,673 or 5,787,211. These techniques require that additional components be added to the system and/or the use of specialty optical fiber that has to be specifically tailored to each length of transmission fiber in the system.
New fiber designs have been developed that substantially reduce the chromatic dispersion of WDM signals during transmission in the 1550 nm wavelength range. However, the decreased dispersion of the optical signal allows for increased nonlinear interaction, such as four wave mixing, to occur between the wavelengths that increases signal degradation. The effect of lower dispersion on nonlinear signal degradation becomes more pronounced at increased bit transmission rates.
A further difficulty in WDM systems is that the signal bit rates are very high and, as a result, electrical cables and optical waveguides need to be precise and uniform in their length and distortion characteristics. However, it is difficult and expensive to manufacture cables and waveguides to such tolerances. One approach to solving this problem has been to manufacture cables and waveguides in varying lengths and to use a xe2x80x9ctrial and errorxe2x80x9d process to find a cable or waveguide that meets the particular needs of a system or component. Such difficulties limit the bit rate of systems and components.
The many difficulties associated with increasing the number of wavelength channels in WDM systems, as well as increasing the transmission bit rate, have slowed the continued advance in communications transmission capacity. In view of these difficulties, there is a clear need for transmission techniques and systems that provide for higher capacity, longer distance optical communication systems.
Apparatuses and methods of the present invention address the above need by providing optical communication systems and components including distortion elements that can provide for dispersion compensation, compensation for other distortion and non-uniformities in the system and components, and/or nonlinear management in the system. In an embodiment, one or more distortion elements are used to compensate for distortions and non-uniformities in electrical cables and optical waveguides. The present invention also includes systems which include one or more distortion elements according to the teachings herein. Furthermore, the present invention includes methods of operating, designing, and assembling a system using the methods described herein.
The distortion elements can change the effective length of the electrical cable or optical waveguide. As a result, the distortion elements can be used to fine tune a component or system to compensate for cables or waveguides that are not sufficiently precise in their length or in some other signal distorting characteristic. For example, if two or more cables need to be the same length so that signals carried by those cables arrive at their destination at the same time, the distortion elements can be used to change the effective length of the cables to provide sufficient precision for the systems and/or components to operate properly.
The distortion elements may be provided in varying sizes, allowing for varying effects on the cable or waveguide with which the distortion element is used. In that manner, it is no longer necessary to manufacture a particular cable or waveguide in varying lengths in order compensate for the imprecision in manufacturing the cable or waveguide. Furthermore, the present invention allows for assembly of systems and components to be performed more quickly and more easily, thereby decreasing the time and cost associated with installing the systems and components.
Accordingly, the present invention addresses the aforementioned problems with providing increased transmission performance of optical systems. These advantages and others will become apparent from the following detailed description.